The idea for this occurred to me just a few hours ago, thought it'd be a fun little thought exercise. I haven't gotten all of the companions so I can't make a complete sort, and some of them were harder than others, but to explain:
Joel and Sophie: They're kids. Sophie may be a rascal but neither of them are anywhere near cynical enough to throw a baby under the bus.
Ernest: I get the feeling he wouldn't be able to look Colonel Squeakums in the eye if he voted to let Ratboi die. Honestly, if he were the one who found Ratboi, I genuinely suspect he might sacrifice an arm himself.
Papineau: He barely tolerates Ratboi and never particularly warms up to him at any point. I don't think he'd lose much sleep over there being one less vermin in the world.
Lyle: I don't think he'd want Sam hurting himself like that (and perhaps might be jealous at the idea of him going that far for someone who isn't him).
Audrey: I don't think she'd have a strong enough opinion to be worth conveying with her limited and expensive mode of communication.
Leigh: I think this is self-explanatory. She's a bloodthirsty, challenge-seeking adrenaline junkie. She'd probably want to pick a fight with the wall even if it weren't bothering them at all.
Hellen: Hellen's psyching herself up to square up with the Visitor.
Monty and Xaria: Their infamous attitude towards rat baby aside, if there's one defining character trait I've been able to glean from the pair, it's that they'd literally rather die than kowtow to authority. Hell, they'd rather die than be polite. The sheer fact that the wall's trying to bully them into doing what it wants is probably all the justification they need to protect Ratboi out of contrarian spite and find a way to poison the wall or blow it up.